The present invention relates to environmentally degradable films comprising a polyhydroxyalkanoate copolymer (PHA)/polylactic acid polymer or copolymer (PLA) blend. Laminates having a first layer comprising a PHA copolymer and a second layer comprising a PLA polymer or copolymer are also disclosed. The films or laminates are used to make disposable articles.
This invention relates to the need for alleviating the growing environmental problem of excessive plastic waste that makes up an increasing volume fraction of materials in landfills. Biodegradable polymers and products formed from biodegradable polymers are becoming increasingly important in view of the desire to reduce the volume of solid waste materials generated by consumers each year. The invention further relates to the need for developing new plastic materials that can be used in applications where biodegradability, compostability or biocompatibility are among primary desirable features of such applications. There have been many attempts to make degradable articles. However, because of costs, the difficulty in processing, and end-use properties, there has been little commercial success. Many compositions that have excellent degradability have only limited processability. Conversely, compositions which are more easily processable have reduced degradability.
A conventional disposable absorbent product is already to a large extent compostable. A typical disposable diaper, for example, consists of about 80% of compostable materials, e.g., wood pulp fibers, and the like. Nevertheless, there is a particular need to replace polyethylene backsheets in absorbent articles with liquid impervious films of compostable material, because the backsheet is typically one of the largest non-compostable components of a conventional disposable absorbent article.
To produce films that have more acceptable end-use properties, choosing acceptable degradable polymers is challenging. The degradable polymers should be thermoplastic such that conventional film processing methods can be employed, including running on converting lines. Further, it is important that the film or large film fragments undergo an initial breakup to much smaller particles during the initial stages of composting.
In addition, there has been an emerging interest in the breathability of disposable hygiene products to minimize the discomfort associated with the accumulation of high humidity. Breathable films that can contain liquid while allowing some passage of moisture vapor are of special interest in constructing such products. Controlling the pore size is achieved by dispersing filler particles uniformly and very finely within the film matrix before a stretching operation. Materials such as polyolefin have such a low affinity to filler surface that it is difficult to obtain a good dispersion of particles. Polyesters have a better affinity to many solid surfaces so that particles tend to spread more easily, however, if the interaction is too strong, the desired mechanical failure at the interface between the filler and film matrix to create pores during the stretching will not occur. Materials with a moderate level of interaction with fillers are needed for breathable films. Further, such materials must be substantially ductile to prevent macroscopic mechanical failure leading to large tears during the stretching. For example, typical aromatic polyesters such as polyethylene terephthalate are too brittle to contain the localized mechanical failure around the individual filler particles.
Polyhydroxyalkanoates (PHAs) are generally semicrystalline, thermoplastic polyester compounds that can either be produced by synthetic methods or by a variety of microorganisms, such as bacteria or algae. The latter typically produce optically pure materials. Traditionally known bacterial PHAs include isotactic poly(3-hydroxybutyrate), or PHB, the high-melting, highly crystalline, very fragile/brittle, homopolymer of hydroxybutyric acid, and isotactic poly(3-hydroxybutyrate-co-valerate), or PHBV, the somewhat lower crystallinity and lower melting copolymer that nonetheless suffers the same drawbacks of high crystallinity and fragility/brittleness. PHBV copolymers are described in Holmes, et al. U.S. Pat. Nos. 4,393,167 and 4,477,654; and until recently were commercially available from Monsanto under the trade name BIOPOL. Their ability to biodegrade readily in the presence of microorganisms has been demonstrated in numerous instances. Due to the slow crystallization rate, a film made from PHBV will stick to itself even after cooling; a substantial fraction of the PHBV remains amorphous and tacky for long periods of time. In both cast film operations and in blown films, residual tack limits processing.
Other known PHAs are the so-called medium to long side-chain PHAs, such as isotactic polyhydroxyoctanoates (PHOs). These, unlike PHB or PHBV, are virtually amorphous owing to the recurring pentyl and higher alkyl side-chains that are regularly spaced along the backbone. When present, their crystalline fraction however has a very low melting point as well as an extremely slow crystallization rate. For example, Gagnon, et al. in Macromolecules, 25, 3723-3728 (1992), incorporated herein by reference, show that the melting temperature is around 61xc2x0 C. and that it takes about 3 weeks to reach the maximum extent of crystallization at its optimal crystallization temperature.
Further poly(3-hydroxyalkanoate) copolymer compositions have been disclosed by Kaneka (U.S. Pat. No. 5,292,860) and Procter and Gamble (U.S. Pat. Nos. 5,498,692; 5,536,564; 5,602,227; 5,685,756). All describe various approaches of tailoring the crystallinity and melting point of PHAs to any desirable lower value than in the high-crystallinity PHB or PHBV by randomly incorporating controlled amounts of xe2x80x9cdefectsxe2x80x9d along the backbone that partially impede the crystallization process. Such xe2x80x9cdefectsxe2x80x9d are either branches of different types (3-hydroxyhexanoate and higher) or shorter (3HP, 3hydroxypropionate) or longer (4HB, 4-hydroxybutyrate) linear aliphatic flexible spacers. The results are semicrystalline copolymer structures that can be tailored to melt in the typical use range between 80xc2x0 C. and 150xc2x0 C. and that are less susceptible to thermal degradation during processing. In addition, the biodegradation rate of these copolymers is higher as a result of their lower crystallinity and the greater susceptibility to microorganisms. Yet, whereas the mechanical properties and melt handling conditions of such copolymers are generally improved over that of PHB or PHBV, their rate of crystallization is characteristically slow, often slower than PHB and PHBV.
In general, however, it has been a considerable challenge to convert these newer PHA copolymers, as well as other biodegradable polymers, into useful forms by conventional melt methods. The polymers remain substantially tacky after they are cooled down from the melt and remain as such until sufficient crystallinity sets in, particularly with PHA copolymers with noncrystallizing component levels above 10 wt %. Residual tack typically can lead to material sticking to itself or to the processing equipment, or both, and thereby can restrict the speed at which a polymeric product is produced or prevent the product from being collected in a form of suitable quality. Consequently, there is a need for an inexpensive and melt processable composition of degradable polymers. Moreover, the polymer composition should be suitable for use in conventional processing equipment. There is also a need for disposable articles made from these films. For breathable film fabrication, there is a need to develop environmentally degradable materials that have a moderate affinity for solid filler surfaces for good particle dispersion and that are soft and ductile to have only localized mechanical failure to create fine pores upon stretching.
Environmentally degradable melt processed blended films comprising a polyhydroxyalkanoate copolymer (PHA) and a polylactic acid polymer or copolymer (PLA) are disclosed. Laminates comprising a first layer comprising a PHA copolymer as described herein and a second layer comprising a PLA polymer or copolymer as described herein are also disclosed. Such blended compositions or laminates generally provide material properties different and improved in any one or more properties as compared to PHA copolymers alone or to PLA polymers or copolymers alone. Properties in which the blended materials or laminates are different and improved are any one of hardness/softness, brittleness/flexibility, tack, stickiness, toughness, ductility, processability, opaqueness/transparency, or breathability, for example. Further, breathable films comprising PHA with particulate fillers are disclosed. Disposable articles comprising the environmentally degradable films are also disclosed.